Outcomes related to traumatic brain injury are thought to be related to cerebral perfusion pressure (among other factors). Cerebral perfusion pressure is equal to mean arterial pressure minus the intracranial pressure. Hypotension decreases cerebral perfusion pressure to the brain and is associated with increased mortality in this patient population. This is of particular importance especially in the prehospital arena where prior research has demonstrated that hypotension is associated with increased mortality in patients with traumatic brain injury. However, there have been no studies to date have examined the relationship between both the depth and the duration of hypotension with mortality in patients with traumatic brain injury. The investigators of this study sought to tackle this very issue.

Methods:

The investigators conducted a retrospective observational study of patients with traumatic brain injury within the EPIC (Excellence in Prehospital Injury Care) database as part of the Arizona State Trauma Registry between January 2007 and March 2014. The primary outcome examined was survival to hospital discharge. Patients were determined to have traumatic brain injury based on trauma center diagnoses as a part of either isolated traumatic brain injury or multi system traumatic injury. More specifically, participants were selected who met the classification for moderate or severe traumatic brain injury based on CDC guidelines, ICD-9 head region severity scores and Abbreviated Injury scores. Patients were excluded from the study if they were younger than 10 years of age, were involved in an interfacility transfer or had any systolic blood pressure greater than 200, or systolic blood pressure of 0 indicating traumatic arrest. Hypotension was defined as SBP <90 mmHg. To calculate the “dose” of hypotension, the investigators looked at the depth of hypotension integrated across exposure time (in minutes) AKA “area under the curve”. The integrated values from all hypotensive segments were added together to obtain a dose (in mmHg-minutes). The relationship between mortality and hypotension dose was examined by logistic regression analysis with adjustment for confounding factors.

Key Results:

A total of 16,711 transports for patients with traumatic brain injury were analyzed during the study period, of which 7,521 met inclusion criteria for the study. The key findings were as follows:

· 539 of 7,521 patients (7.2%) were hypotensive during transport

· Among patients with no hypotension (6,982 patients), mortality was 7.8% (95% CI 7.2 to 8.5%). This compared to patients who were hypotensive, where there was 33.4% (95% CI 29.4 to 37.6%) mortality

· Mortality increased in a linear relationship using a log2 hypotension dose and log odds of death (OR =1.19, 95% CI 1.14 to 1.25) per 2 fold increase in hypotension dose increase. In specific quartiles of hypotension dose, the following outcomes were established:

o 16.3% mortality with dose between 0.01 to 14.99 mmHg-minutes

o 28.1% mortality with dose between 15 to 49.99 mmHg-minutes

o 38.8% mortality with dose between 50-141.99 mmHg-minutes

o 50.4% mortality with dose greater than 142 mmHg-minutes

Takeaways:

· A dose response exists between prehospital hypotension dosage and mortality. Each 2 fold increase in hypotension dose (depth of hypotension integrated over time) during prehospital transport is associated with a 19% increase in mortality

What this means for EMS:

Out-of-hospital hypotension for patients with traumatic brain injury is associated with worse patient outcomes, i.e. decreased survival to hospital discharge. While this study was observational and did not address whether treatment of hypotension improved survival or neurologically intact recovery, it did emphasize an important variable that may serve as the foundation for future EMS research and quality improvement initiatives regarding the management of traumatic brain injury in the field. Going forward, more accurate (and more frequent) acquisition of blood pressure measurements in the prehospital setting may prove to be invaluable in implementing future prehospital resuscitative strategies for patients with traumatic brain injury.